Found via Mebane Faber.
Ben Graham liked to distinguish between a temporary loss of value and a permanent loss of capital. The former is a rebalancing opportunity; the latter is a disaster. In a highly diversified portfolio where all the idiosyncratic risk has been diversified away, the latter is extremely rare. At some time during the 20th century, the stock markets of Argentina, Russia, Germany, Japan, China, and Egypt each went essentially to zero. Suffice it to say those investors had much bigger things to worry about than their stocks! Temporary losses of value are frequent; at times they can become so frightening that they become permanent—for those that sell.